As is set forth hereinbelow, the present invention combines quadrature amplitude modulation techniques with partial response coding techniques resulting in modems which provide unusually high quality digital transmission at specific data rates. Before considering the invention itself, quadrature amplitude modulation and partial response coding techniques will be briefly considered by way of background.
Quadrature Amplitude Modulation (QAM) techniques have been used by data communication engineers for some time. In accordance with this general technique, the data to be transmitted is split into inphase (I) and quadrature phase (Q) channels. After suitable shaping (band-limiting), the signal in each channel is modulated with inphase and quadrature carriers, respectively. The modulated signals in the two channels are then summed to produce a band-limited QAM signal. In general, the error rate performance of a data communication using QAM techniques is less susceptible to small amounts of phase jitter in the demodulating carrier than systems using vestigial sideband (VSB) or single sideband (SSB) modulation techniques. In a high performance VSB or SSB system, a portion of the carrier and pilot tone is transmitted with the information bearing portion of the signal in order to permit recovery of the carrier. In a QAM system, all of the transmitted power is allotted to the information bearing portion of the signal, and the carrier can be recovered from the quadrature amplitude modulated signal itself (see Stiffler, "Theory of Synchronous Communications", Prentice Hall, 1971). Reference is also made to, e.g., Lucky, Salz and Weldon, "Principles of Data Communication", McGraw-Hill, 1968, for a further description of QAM systems.
Partial response techniques, wherein a controlled amount of intersymbol interference is employed in order to increase the transmission rate, are a more recent development and have been used with an AM-SSB scheme to achieve certain spectral shaping effects in data transmission. Reference is made to, e.g., Kretzmer, "Generalization of a Technique for Binary Data Communication" IEEE Transactions on Communication Technology, February 1966, for a description of various classes of partial response schemes and precoding techniques, such precoding being used to avoid error propagation in the decoded data. Reference is also made to U.S. Pat. No. 3,388,330 (Kretzmer). For a binary input signal, the partial response baseband signal includes at least three levels. A partial response system requires a higher signal-to-noise ratio than an ideal binary system for the same error rate. However, since the number of levels in a partial response system is always higher than the number of input levels, certain sequences of the estimated data can be eliminated as being impossible and thus a system using a three-level (ternary) partial response baseband signal can perform substantially as well as an ideal binary system. Reference is made to U.S. Pat. Nos. 3,492,578 (Gerrish et al) and 3,679,977 (Howson) for descriptions of multilevel precoded partial response data transmission systems.